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I have recently done some testing to optimize the intake cam timing of a fresh motor. My main objective was to find the timing that gave the best acceleration times under controlled conditions but an interesting piece of information is the effect of the timing on the AFR curve.

The motor is a 2L with a Euro manifold and 45DCOE152's, RJ785 exhaust cam which was timed with a LC of 105 and an RJ575 intake cam timed at 99, 102, 105 and 108. 102 and 105 gave almost identical times, 99 was a bit slower and 108 was significantly slower.

What I find interesting is how the rich dip at 4500 rpm changes with LC. It is deepest and broadest at 99 and shallowest and narrowest at 108. Jim Steck says that the rich dip is due to reversion in the intake and that is greatest with the latest closing of the intake valve.

The screen shots of the AFR/RPM logs are shown in the order 99, 102, 105, 108.


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